Abstract: The microdosimetric spectrum and radial dose distribution of 16 O + particles with energy 109 keV generated from small low energy heavy ion accelerator permeating through PET film (C 10 H 8O 4) with thickness of 4 μm are measured by the cylindrical tissue equivalent proportional counter. The results show that as a function of the radial distance b from trick core, the dose mean specific energy of single event z iD is inversely proportional to square of the radial distance. The dependence of z iD on intensity of 16 O + particles after passing through PET film with varies thickness are measured. One discovers that thickness of PET plastic film permeated through by 16 O +1 particles with energy of 87.5～119 keV is 10 times more than calculated ranges by TIRM92 Monte Carlo program. The results are compared and discussed.